Power supplies and especially power converters are an important and rapidly expanding technology that impacts a broad range of applications including computer systems and telecommunication environments. A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a stable and well-regulated output, switched-mode power converters are frequently employed to advantage. A typical power converter operating in a switched-mode fashion may include an EMI filter, an inverter, a transformer having a primary winding coupled to the inverter, an output circuit coupled to a secondary winding of the transformer, an output filter and a controller. The inverter generally includes a power switch, such as a field-effect transistor (FET), that converts an input voltage to a switched voltage that is applied across the transformer. The transformer generally transforms the voltage to another value. The output circuit generates a desired voltage at the output of the converter and typically includes an output capacitor that smooths and filters the output voltage for delivery to a load.
Power converters must sometimes accommodate severe transient conditions and operate within a specific range of input voltages. During a start-up mode, a transient condition of particular interest occurs at the time that an input voltage is first applied to a power converter. If an AC power source is applied at a time near its peak voltage, a large inrush current transient may cause damage to components of the power converter. This condition may be avoided by the use of an inrush limiting resistance that limits the maximum value of the inrush current transient to an acceptable level. The inrush resistance, however, must be removed from the circuit during normal operation to preserve the overall efficiency of the power converter. Ideally, the inrush resistance should be placed on the input voltage side of the power converter between the EMI filter and the inverter to protect the power converter from the transient condition and to prevent EMI noise from emitting out into the source of input voltage by bypassing the EMI filter.
The power supply controller typically monitors the input voltage to determine if and when the input voltage has reached a required value both to start and to sustain the operation of the power converter. The ideal position for monitoring the input voltage is between the EMI filter and the inrush resistor. The controller typically monitors the peak value of the AC input voltage to determine when the required value is reached. If the inrush resistance is placed at the input of the power converter, (i.e., before the EMI filter) to maximize component protection, the peak of the AC input voltage waveform may be sufficiently distorted or reduced on the output side of the inrush resistance to cause the controller to misinterpret the magnitude of the AC input voltage. Such a misinterpretation may cause the power converter either not to start or to sustain its operation.
Accordingly, what is needed in the art is a system and method employable with a peak-mode controller to accommodate average signals thereby overcoming the deficiencies of the prior art, such as those described above.